This invention relates to a solar cell power supply circuit.
Lately, with the advent of very low power LSI elements, solar cell powered equipment such as solar cell calculators and wristwatches have been in increasing use to fulfill the demand for savings of power. However, this equipment, for example, solar cell calculators has the disadvantage that information will disappear during the process of calculation when incident light to a solar cell or cells is blocked. There are two measures to overcome this disadvantage: (1) a back-up capacitor is connected in parallel with the solar cells to temporarily protect operation of the LSI element or the solar cells, load especially when incident light to the solar cells is blocked; and (2) a back-up battery or batteries are installed in the equipment to energize the same when incident light is shut off. Those measures have been considered as unsatisfactory as follows.
In the circuit method (1) (see FIG. 1)
As shown in FIG. 1, the back-up capacitor C.sub.1 is provided in parallel relationship with the solar cells SB and serves to compensate for interrupted incident light to the solar cells SB. There are further provided voltage-stabilizing LEDs (D.sub.1 and D.sub.2) in parallel relationship with the capacitor C.sub.1 to prevent the LSI element from being suppled with an overvoltage when a very large amount of incident light is applied to the solar cells SB. In other words, the LEDs D.sub.1 and D.sub.2 stabilize the output voltage of the solar cells SB. In this circuit, when the output voltage of the solar cells SB reduces to zero the capacitor C.sub.1 makes up for a deficiency of voltage necessary for the driving of the equipment when the incident light to the solar cells SB is blocked. The back-up capacitor C.sub.1 therefore protects operation of the LSI element when the incident light is shut off. To lengthen the operational life of the equipment, it is necessary to employ a back-up capacitor C.sub.1 having a capacitance as high as possible. Nevertheless, it takes a long time to make the equipment ready to operate after the equipment is removed from the dark and subjected to solar or other radiation. That is, in the event that the equipment is moved somewhere from the dark with no charge on the capacitor C.sub.1, it will take a considerable amount of time for the output voltage of the solar cells to reach a voltage level necessary to enable the voltage at the back-up capacitor C.sub.1 to drive the LSI element and the operator is unable to use the equipment for this period of time. The length T.sub.1 of time required for making the equipment ready to operate (hereinafter this is referred to as "recovery time") can be defined approximately as follows: EQU T.sub.1 =(C.sub.1 .multidot.V)/(I.multidot.A) (1)
where
C.sub.1 the capacitance of the back-up capacitor PA0 A the brightness of the incident light to the solar cells PA0 the output current of the solar cells illuminated with the brightness A
V the voltage to be supplied to the LSI element for normal operation of the equipment
Equation (1) indicates that shortening the recovery time T.sub.1 requires decreasing C.sub.1 and V and increasing I and A, in which case such a decrease in C.sub.1 causes deteriorating the primary performance of the back-up capacitor and such a decrease in V leads to a yield drop and cost increase in the manufacture of the LSI elements. Further, an increase of A limits correspondingly the range of brightness usable with the solar cell-powered equipment and an increase of I demands an increase in the area of the solar cells. The solar cells are generally more expensive than conventional batteries and the requirement for an increase in the area of the solar cells leads to greater expensive.